One particularly effective optical amplifier is described in U.S. Pat. No. 7,256,931. This amplifier includes a slab-type gain-element of a highly doped gain-medium such as neodymium-doped yttrium vanadate (Nd:YVO4). The gain-element is energized by laser-radiation from a linear array of diode-lasers (semiconductor-lasers) commonly referred to by practitioners of the laser art as a diode-laser bar.
Diode-lasers in a diode-laser bar have emitters characterized as having a fast-axis, in which radiation is emitted with a relatively high divergence (for example up to about 35° half-angle), and a slow-axis in which radiation is emitted with relatively low divergence (for example, less than about 10° half-angle). The emitters are aligned, spaced apart, in the slow-axis direction. Typically, a diode-laser bar has a length of about 10 millimeters (mm) and width between about 1 and 2 mm. Emitter apertures have a slow-axis dimension of up to about 100 micrometers (μm) and a fast-axis dimension of about 1 μm. One preferred number of emitters per bar is 19.
In an optical pumping arrangement for a conventional slab-amplifier, radiation from the diode-laser bar is collimated in the fast-axis by a first, elongated, cylindrical lens referred to as a fast-axis collimating (FAC) lens. The fast-axis collimated radiation is then focused by a second cylindrical lens into a lateral surface of the gain-element. The radiation is primarily absorbed close to the lateral face to form a narrow, shallow, nominally linear gain-region at that lateral face.
A beam of radiation to be amplified is directed into the gain-element through a first end-face thereof into the linear gain-region at a first location such that the radiation is incident at grazing incidence, for example at an incidence-angle of about 80°. The radiation is reflected by total internal reflection (TIR) out of the gain-element through an opposite second end-face thereof. That radiation is then directed by a pair of mirrors back into the gain-element through the second end-face back into the linear-gain region, again at grazing incidence, but at a second location spaced apart from the first location. TIR directs the twice-incident (amplified) radiation out of the gain-element through the first end-face thereof.
Emitting apertures in a diode-laser bar are rarely exactly fast-axis aligned. Misalignment in the fast axis direction can occur in particularly when the diode-laser bar is mounted on a heat sink. This fast-axis misalignment, which can be as much as 20 μm, is whimsically referred to by practitioners of the art as “smile”. This presents a problem in the above-discussed optical pumping arrangement inasmuch as any smile in a diode-laser bar will be reproduced and magnified by the collimating and focusing lenses in the nominally linear gain-region of the gain-element. This can result in inefficiency or inconsistency of the amplifier gain.
If such inconsistency and inefficiency cannot be tolerated, diode-laser bars having tolerably low smile can be selected from a batch of mounted diode-laser bars. This of course can result in low yield and consequently higher cost for the selected mounted bars. There is a need for an optical pumping arrangement which is insensitive to smile in a diode-laser bar.